Light-emitting elements (organic EL elements) including organic compounds and utilizing electroluminescence (EL) have been put to more practical use. In the basic structure of such light-emitting elements, an organic compound layer containing a light-emitting material (an EL layer) is interposed between a pair of electrodes. Carriers are injected by application of voltage to the element, and recombination energy of the carriers is used, whereby light emission can be obtained from the light-emitting material.
The light-emitting elements are self-luminous elements and thus have advantages over liquid crystal displays, such as high visibility and no need for backlight when used as pixels of a display, and are suitable as flat panel display elements. In addition, it is also a great advantage that a display including such light-emitting elements can be manufactured as a thin and lightweight display. Furthermore, extremely high response speed is also a feature thereof.
In such light-emitting elements, light-emitting layers can be successively formed two-dimensionally, so that planar light emission can be obtained. Thus, a large-area light source can be easily formed. This feature is difficult to realize with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applied to lighting devices and the like.
Displays or lighting devices including light-emitting elements can be suitably used for a variety of electronic devices as described above, and research and development of light-emitting elements have progressed for higher efficiency or longer lifetimes.
An organic acceptor is given as an example of a material of a hole-injection layer that is used for facilitating injection of carriers, particularly injection of holes, to an EL layer. An organic acceptor is suitable for mass production because it can be easily formed by evaporation; therefore, the use of an organic acceptor spreads widely. However, when the LUMO level of an organic acceptor is distanced from the HOMO level of an organic compound included in a hole-transport layer, it is difficult to inject holes to an EL layer. Thus, in order to make the LUMO level of the organic acceptor closer to the HOMO level of the organic compound included in the hole-transport layer, a substance with a shallow HOMO level is used as the organic compound included in the hole-transport layer. In that case, the difference between the HOMO level of a host material used in the light-emitting layer and the HOMO level of the organic compound included in the hole-transport layer is large. Therefore, even when holes can be injected to the EL layer, it is difficult to inject holes from the hole-transport layer to the host material of the light-emitting layer.
Patent document 1 discloses a structure in which a hole-transport material whose HOMO level is between the HOMO level of a first hole-injection layer and the HOMO level of a host material is provided between a light-emitting layer and a first hole-transport layer in contact with the hole-injection layer.
Although characteristics of light-emitting elements have been improved remarkably, advanced requirements for various characteristics including efficiency and durability are not yet satisfied.